Features of Slope-Movements due to Heavy Rainfalls in the SHIRASU Region of Southern Kyushu

Baiu-fronts and typhoons passing through southern Kyushu and its adjacent area often bring about extraordinarily heavy rainfall, and, consequently, result dangerous floods and collapses of hill-slopes with Shirasu. Shirasu, which is widely distributed over souhern Kyushu, is very susceptible to water erosion and ready to collapse. This paper deals with the failure of hill-slope that occurs in the period of heavy rainfall. In this paper, the author classifies the failure of hill-slope into ten basic types according to the features of slope-movements and describes their causes for failures briefly. The hill-slopes with Shirasu are likely to be led to the most frequent collapses in the event of the rainfall exceeding about 300 mm during a spell of rain and in less than several hours after the time when the maximum hourly rainfall has occurred